1. Field of the Invention
The present invention relates to a rotary type electrostatic spray painting device.
2. Description of the Related Art
In a known rotary type electrostatic spray painting device, a rotary shaft thereof is supported by static pressure air bearings within the housing of the device. A spray head is fixed to the outer end of the rotary shaft, and a turbine wheel for driving the rotary shaft is fixed to the inner end of the rotary shaft. An air injection nozzle which provides a rotational force in one direction for the turbine wheel is arranged in the housing of the device to provide a driving force for rotating the rotary shaft (hereinafter referred to as the drive nozzle), and another air injection nozzle which provides a rotational force in the other direction for the turbine wheel is also arranged in the housing of the device to provide a braking force for the rotary shaft (hereinafter referred to as the brake nozzle). Air under pressure is fed into the static pressure air bearings (hereinafter referred to as the air bearings) so that the rotary shaft is supported by these air bearings in a non-contact state. When a painting operation is carried out, air under pressure is injected towards the turbine wheel from the air injection drive nozzle, and the rotary shaft is thus rotated at a high speed. When the painting device is to be cleaned, to enable a change of the color of the paint to be sprayed, air under pressure is injected towards the turbine wheel from the air injection brake nozzle so that the rotating speed of the rotary shaft is quickly reduced (Japanese Unexamined Utility Model Publication No. 58-124254).
In this rotary type electrostatic spray painting device, a certain amount of air under pressure is necessary to support the rotary shaft by means of the air leavings in a now-contact state, and to keep this pressure to a minimum, the clearance between the bearing faces of the air bearings and the outer circumferential wall of the rotary shaft is very small, i.e., about 20 .mu.m to 70 .mu.m. In addition, to prevent a seizure between the rotary shaft and the air bearings, a surface hardening treatment such as flame spraying ceramic material and the like is applied to the outer circumferential wall of the rotary shaft.
However, in this rotary type electrostatic spray painting device, since the amount of air fed into the air bearings is relatively small, the force supporting the rotary shaft is weak. In addition, the clearance between the bearing faces of the air bearings and the outer circumferential wall of the rotary shaft is very small as mentioned above. Therefore, if an insufficient amount of pressurized air is fed into the air bearings when the rotary shaft is rotated, or when the rotating speed of the rotary shaft is quickly reduced, the rotary shaft comes into contact with the bearing faces of the air bearings during rotation, and as a result, when the rotating speed of the rotary shaft is low, although there is no danger of a seizure between the rotary shaft and the bearing faces of the air bearings, a problem occurs in that a large frictional force is generated between the rotary shaft and the bearing faces of the air bearings, and thus the rotating speed of the rotary shaft is reduced. When the rotating speed of the rotary shaft is high, even if a surface hardening treatment has been applied to the outer circumferential wall of the rotary shaft, a seizure will occur between the rotary shaft and the bearing faces of the air bearings, and thus a problem occurs in that the rotary shaft cannot be rotated.